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Month: December 2017

Filippo Valsorda wrote an article “How Plex is doing https for all its users” two years ago. In the article, Filippo explains how the self-hosted media server Plex can offer TLS to secure all connections, including those to the user’s servers.

Plex is a server software running on your machine, and a discovery service somewhere out on the internet. Using your login, you connect to the discovery service, and then connects directly to your server, using XHR.

The XHR part means you are on a page https://plex.tv/web.app, and because that is a https page, the XHRs also need to be encrypted and trusted. That means your server needs to be able to do https, and that means your server needs to have a valid certificate to do this.

So today is not only a day with a snowflake coming down and touching the ground, which in itself is always an event that puts the Netherlands into chaos (The Dutch are generally awesome with liquid water, but are completely unable to deal with the same substance in a frozen state).

Our annual day of winter.

Today is also the day where Connexxion switches to the new Dienstregling, stops accepting cash in the buses and also switches to new equipment. This is not without it’s share of problems.

Frank O’Brien explains an outstanding piece of engineering: The Apollo Guidance Computer (AGC) was the computer that controlled the thrusters and navigation on the Apollo Command Module, and a second instance, on the Lunar Module.

It’s a 32kg box with core memory and core rope ROM, a 16 bit discrete CPU (15 data bits and parity) running at 1 MHz effective, and some very special hardware behind its I/O ports. Memory was 2k words of writeable core memory, and some 36k of ROM.

The processor architecture is stackless, which complicates many things, and despite being designed in 1966, already has multiplication and division machine instructions. Addresses are effectively 12 bit, so sophisticated bank switching to extend the address space is needed, writeable and read-only memory are banked separately. Some interesting interaction with interrupt processing and bank switching exists.

There exists a very basic multitasking system, which at the same time is made simpler and more complicated by the stackless architecture. On top of the multitasking executive sits the interpreter, a virtual machine that was much more comfortable than the actual hardware and had a stack and index-registers, as well as trigonometric math functions. Navigational code ran on the interpreter, basic hardware control (thrusters, sextant, inertial control system and anything timer related) ran on the executive.

The book gives a guided tour into a hardware design over fifty years old, which itself was already foreshadowing and in some way surpassing the more modern integrated chip CPUs of seventies home computers. It also demonstrates a number of creative and interesting ideas of how to achieve remarkable things with very little hardware.

The Apollo Guidance Computer: Architecture and Operation
Frank O’Brien
EUR 30.71 (purchased as Kindle edition, no longer available as eBook)Paperback

A few days ago, a few of “us” have been to france. On a cold November morning in a brand new data center hall, we had a look at some Version 1 OCP racks, and a very nice conversation with a bunch of friendly people interested in getting the foundation going.

A OCP Version 1 rack, with three power zones. You can see the centralized power supplies at the bottom of each of the zones.

See Open Rack Specs and Designs, the Open Rack Standard 1.2 Spec and Facebook Open Rack V1 Specification. There is also the Facebook V1 Power Shelf Specification.

Tom Randall has a Twitter thread on things that affect battery price for LiIon battery packs of electric cars. »If lithium prices quadruple, the cost of finished battery packs would only go up 1.6%, and the sticker price of an electric car will see almost no impact. Some people may get rich off metals in the meantime, but it’s not going to slow down the transition to electric cars.«